Core-type furnace

ABSTRACT

Vertical segments of a side wall of a crucible for an induction furnace are assembled at an adjustable, invariable position by screws screwed into tapped holes of a flange common to all segments. A precise assembly is thus obtained producing no deformation and no internal stresses. The segments are coated with a ceramic coating for their protection and to prevent formation of electric arcs. Junction edges of faces are rounded to achieve the same effect. Water cooling boxes of the lower furnace hearth are similarly constructed. The apparatus can, as an example, be applied to vitrification techniques.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The subject matter of this invention is an induction furnace crucibleand hearth for the incineration and vitrification of organic matter, thevitrification of radioactive and non-radioactive waste, thevitrification of hazardous waste and the fusing of refractory bodies.

2. Description of the Related Art

The structure of said furnaces essentially comprises a hearth inrefractory concrete including cooling water circuits on which a sidewall is arranged called the crucible, surrounded by an inductive coil inwhich an electric current circulates at frequencies higher than 100 kHzwhich is the source of the power produced inside the crucible to meltthe matter therein. These furnaces are chiefly used for the incinerationand vitrification of organic matter, the vitrification of radioactive ornon-radioactive waste and the fusion of refractory bodies. Theindustries likely to have recourse thereto are waste treatmentindustries including nuclear and hazardous waste treatment, and theglass industry.

The side wall of the crucible is normally in metallic material permeableto magnetic fields. It contains a cooling circuit so that, firstly, thewall can resist the very high temperatures reached to melt refractorymaterials such as glass and, secondly, it can compensate for theelectric power dissipated by the joule effect within the structure. Saidcrucible is called a “cold crucible”. In addition it is normally dividedinto vertical segments, joined by their transverse faces by interposingan electric insulation material to limit currents induced in the wallwhich would cause heat losses and electromagnetic coupling between theinductor and the content of the crucible. The vertical segments arearranged similar to barrel staves. The cooling circuit usually consistsof vertical channels bored in each of the segments.

The segments of the side wall of the crucible must be held together. Onefirst means consists of surrounding the crucible with circular bandingin cement or glass fabric impregnated with elastomer or epoxy resin.Another means, offering greater cohesion, consists of welding thesegments to one another on a circular flange above the inductor wherethe intensity to magnetic fields is lower. A last type of assembly whichis preferred for the invention consists of assembling the verticalsegments and forming the ferrule by screws on a circular flange abovethe inductor. To facilitate assembly, the segments are provided withassembly lugs on the part mounted outside the ferrule.

The hearth supporting the ferrule is made up of metallic boxes throughwhich a cooling circuit passes, the boxes being placed in refractoryconcrete, or consists of metallic tubing of various section (round,square, rectangular etc.) mounted in parallel or in chevrons and placedin refractory concrete. The boxes or tubes are separated from oneanother by a width of refractory concrete. One of the faces ispositioned so as to lie perfectly opposite the content in fusion withinthe furnace. Similar to the tubing, the boxes may be of diverse shape:rectangular, triangular etc.

Known crucibles and hearths suffer from deficiencies which can bedetailed as follows. For application to the combustion-vitrification oforganic matter over molten glass baths, or the fusion of refractorybodies in an induction furnace, the frequencies and heats required aremuch higher than for other applications. Risks of electric shortcircuits may occur between the metallic elements forming the coldcrucible (segments, flanges), forming the hearth supporting the crucible(cooled metallic boxes) and between the parts of the crucible and thehearth. These short circuits occur even when the electric insulationplaced between the crucible segments and the hearth cooling boxes is oflarge width.

Without being exhaustive, these electric short circuits between thecrucible segments and the hearth boxes are possible through the presenceof carbon deposited on the inner walls during the combustion of organicmatter, or through the formation of pools of sulphates on the surface ofthe glass baths entering into contact with different segments and theelectric insulations in the inter-segments, or for example through therelease of a large quantity of water at the time of fusion of refractoryoxides. These short circuits cause irremediable damage to the electricinsulations positioned between the parts forming the crucible, to therefractory concrete placed between the hearth cooling boxes, or can evenpierce the metallic elements of the hearth and crucible. These electricshort circuits are also harmful to efficient use of induction energy.

In the aforesaid applications, corrosive atmospheres at hightemperatures are produced, which damage the metallic parts of thefurnace consisting of the crucible and hearth, or require the same to bebuilt in materials having high electric resistivity, considerablyincreasing electric losses.

Irrespective of the shape of the crucible segments (parallelepiped,T-shaped, triangular.) and of the hearth, the sharp edges of theseadjacent metallic parts are the source of substantial electric arcs(electric spiking effect). Operating schedules are the chiefcontributors towards this onset of electric arcs, schedules demandingfrequencies greater than 100 kHz for glass applications and wastetreatment over molten glass baths. These electric arcs are energetic andharmful to the resistance of the electrical insulations of the crucibleand hearth concrete. It is specified that if the crucible segments wereround or ovoid, this would eliminate spiking effects but to thedetriment of the imperviousness of the furnace ferrule by reducing toofar the thickness of the electric insulation between the segments, whichwould lead to problems of matter and gas leakages as soon as theinsulating material shows slight deterioration.

It is to overcome these disadvantages that a new type of crucible andhearth for an induction furnace is put forward as the invention.

SUMMARY OF THE INVENTION

To avoid the occurrence of electric arcs, the solution chosen consistsof coating the metallic segments forming the crucible and metallic boxesof the hearth on one or all their faces with a ceramic electricinsulation layer: at least on the inner and side surfaces of thesegments facing one another to eliminate electric arcs or, dependingupon chemical and electrical attack, on all the faces including thehead, foot and surface facing the exterior of the furnace. These ceramiccoatings are provided in addition to the electrical insulation placedbetween the segments of the crucible and the hearth boxes, and theyprovide perfect electric protection between the different metallicelements of the furnace and even between the metallic elements and thecoating under fusion. In addition, thus coated, the segments of thecrucible and the hearth boxes are protected against chemical attack dueto glass, gases and other different waste fed into the cruciblesupported by the hearth. Refractory ceramic coatings, which are perfectelectric insulators, are made by acetylene torch for example or plasmatorch. The materials the most frequently sprayed contain alumina,mullite, cordierite, zircon, zirconia, silicon zirconate and carbide,with various dopants compatible with electric stresses.

Once coated on one or all their faces, the metallic boxes are placed inthe hearth interposing an electric insulator such as refractoryconcrete. As for the crucible segments, once coated on one or all theirfaces with ceramic electric insulation, these may be mounted and screwedonto the cooled flange which may also be coated with electricinsulation. In the description of the invention details will be given ofthe screw-mounting of the crucible which limits mechanical assemblystresses (local compressions) and heat stresses (if there are welds) butthe invention can be fully applied to other types of assembly detailedin the prior art.

In the literature it is found that it is preferable to chamfer the sharpedges to avoid weakening of the ceramic coating and its flaking. While achamfer on the sharp edges of the segments may help towards satisfactorydepositing of the ceramic electric insulation on the segment faces, thisis not at all sufficient to withstand the occurrence of electric arcs atfrequencies above 100 kHz between the hearth boxes and the faces ofthose segments forming the inner part of the crucible, which for examplelie opposite the carbon dust derived from the combustion of organicmatter over the molten glass bath or opposite the elements to bevitrified.

The sharp edges oriented towards the inner surface of the furnace arerounded to a radius of curvature. The elimination of all sharp edgesthrough radius of curvature machining concerns the sharp edges facingthe inside of the induction furnace. The presence of chamfers on theother sharp edges outside the crucible may be sufficient without beingobligatory. The size of these radii of curvature gives the followingoperating functions:

-   -   the radius of curvature must not be small (less than 1 mm for        example) to avoid any matter being trapped in the free air gap        between the segments when the height of the glass bath varies,    -   as in some configurations described in the prior art, an        electric insulator such as mica may be maintained in the        inter-segment space (mica thickness of between 0.1 or 4 mm) or        the connection elements may be mounted with no additional        electric insulation other than the ceramic deposit. The radius        of curvature must be low (less than 5 mm) to ensure that the        cooled metallic segments are sufficiently close to prevent the        molten glass from coming into contact with the electric        insulation placed in the spaces, which could deteriorate this        insulation and allow matter to leak out from the crucible.

The invention sets itself apart in the specific cases concerning theincineration and vitrification of organic matter, the vitrification ofwaste and the fusion of refractory bodies, through its low heat flowexchange rates between the matter to be vitrified and the furnace walls.By way of example, these flow rates are lower by one order of magnitudethan in cold crucibles for metal fusion through the self-generation,against the furnace wall, of a shell of glass that is solid andrefractory. Under these conditions, the ceramic materials for electricprotection are perfectly cooled preventing their deterioration, theirflaking and above all preventing pollution of the vitrified matter.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail and under everyaspect in connection with the figures:

FIG. 1 shows a welded crucible according to the prior art,

FIGS. 2 and 3 illustrate an embodiment of a crucible of the invention,

FIGS. 4 and 5 illustrate the mode of fabrication of the crucible and

FIGS. 6 and 7 illustrate a hearth of the invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, a crucible comprises a hearth in refractoryconcrete which carries reference 1, a side wall carrying reference 2,its segments in stainless steel carrying reference 3, intermediatelayers of electric insulation reference 4, and inductor coils reference5. The details of construction and arrangement of these parts complywith the aforesaid description. Side part 2 is only partly shown, but itis clear that it extends over a circle or complete turn as for any othercrucible including those of the invention. A cooling circuit 6 ishollowed out of each of segments 3, which extends over practically theirentire height and is here made up of a pair of parallel ducts meeting atthe bottom of segments 3 (only one of these ducts being visible in thecross-section). By means of pierced inlets and outlets 7 and 8 for thecooling liquid, the ducts communicate outside of segments 3 and lead tosuperimposed collectors 9 and 10 belonging to the same flange 11 towhich segments 3 are welded by a circular bead 12 on their top outeredge. Even with this welding, it is possible to add to the structure anouter banding 13 under flange 11 to improve the cohesion of the sidewall 2 and to ensure a gas seal. The disadvantages mentioned aboveconcerning the two assembly modes for the side wall 2 are not eliminatedeven if these modes are combined. Hearth 1 is cooled by the circulationof water in the metallic boxes which have not been shown in this figure.

An embodiment of the invention will now be described with the help ofFIGS. 2 and 3.

The segments of the side wall carry reference 20. They have the sameouter shape and similarly have a pair of ducts passing through them as acooling circuit 21 whose ends lead to the outside via tubes 23 a and 23b (FIG. 3). But contrary to the prior art, segments 20 of the inventionare not bare but are coated with a ceramic coating 22 which may bechosen from among compositions containing alumina, mullite, cordierite,zircon, zirconia or zirconates, different additives optionally beingadded in relation to the thermal, chemical and electric stresses whichthe crucible may have to undergo. A single segment 20 is shown withcoating 22 in FIG. 2, but all the segments are coated. Similarly coating22 is present on segment 20 in FIG. 3 but has not been shown for reasonsof clarity. It is recommended to coat at least the inner face 24 ofsegments 20 and their side faces 25 and 26, which are the facessubjected to corrosion and the onset of electric arcs; however, it wouldalso be expedient to coat the outer face 27, as shown here, or even thetop and bottom faces. Since chemical attack or risks of electric shortcircuits which could warrant the use of coating 22 would derive from thegases staying above the molten matter and from the particles andreleases carried by these gases rather than from the molten matteritself, one of the functions of these cold crucibles being to maintain asolid thickness of the crucible content on the side wall, coating 22extends as far as the top of segments 20. Its thickness lies between 50μm and 500 μm depending upon applications. One additional arrangement toreduce the probability of electric arcs while allowing better adherenceof coating 22, is to eliminate the sharp edges between faces 24 to 27 ofsegments 20: here the sharp edges 28 and 29 on the inside of the furnace(between the inner face 24 and side faces 25 and 26) have been roundedto a radius of curvature of possibly one to five millimetres, and theother sharp edges such as 30 and 31 (between the outer face 27 and theother side faces 25 and 26) have simply been chamfered; this latterarrangement is only necessary to facilitate the adherence of coating 22to the junction of the two coated faces. The horizontal sharp edges ofsegments 20, at the top and bottom, may also be rounded or chamfered ifelectric arcs are a risk with neighbouring elements.

With special reference to FIG. 3, it can be seen that flange 11 hasdisappeared and that the cooling circuits 21 are not associated withcollectors such as 9 and 10 adjacent to the crucible but are completelyseparate, tubes 23 a and 23 b extending to the outside. Segments 20comprise an upper lug 32 also in a sector of circle which overhangs theouter face 27. It comprises a cut-out 33 opening onto the outside. Aflat flange 34 of circular shape is laid on all lugs 32 and comprisestapped holes 35. Screws 36 are engaged in tapped holes 35 throughcut-outs 33 and lean against the underside of lugs 32 holding themagainst the flat flange 34. Therefore the segments 20 are held inposition and form a single assembly. An outer banding 37 may be added toensure an air seal for the crucible and render the assembly more solidbut is not indispensable; it may be in solid glass fabric impregnatedwith elastomer or epoxy resin. Finally, layers of electric insulation 38in mica for example may be inserted between the side faces 25 and 26 ofneighbouring segments 20.

A ceramic coating 57 may also be deposited on flange 34, and above allon its lower face 58 touching lugs 32 of segments 20. Here again it isexpedient to chamfer the sharp edges joining two faces coated withceramic.

Another arrangement, made possible through flat flange 34, consists ofadding a cover 39 laid on the flange and held by two clamps 40 withscrews 41 engaged in tapped holes of the flat flange 34 so as to confinethe content of the crucible and ensure a perfect seal.

It has already been mentioned that with the precise, invariableadjustment of segments 20 made possible through an assembly using screwsand a flat flange 34, the segments 20 can be coated with ceramic withoutany risk for the ceramic. A method for assembling the side wall will nowbe described with which it is possible not to expose the ceramic todamage even with this configuration; this description will be given withreference to FIGS. 4 and 5. Segments 20, after being sufficientlyprecision machined at the required points (in particular at the lowerface, laid on concrete hearth 1, at the upper face of lugs 32 and atside faces 25 and 26) and coated with ceramic by plasma deposit andabrasive polishing, are roughly positioned on the flat flange afterbeing turned around, a conical centring wedge 42 is placed on them andclamp collars 43 are inserted around them and tightened to bring theminto contact with the entire conical flank of wedge 42. The layers ofelectric insulation 38 have already been inserted. Depending upon theheight of wedge 42 and the clamping of collars 43 the diameter of theside wall and its preload can be adjusted. Screws 36 are then tightenedto contact lugs 32 with underlying flat flange 34. The assembly is thencomplete. Banding 37 may be formed firstly by wrapping 371 placedbetween the clamp collars 43, then by additional wrapping when the clampcollars 43 have been removed. This two-step laying of the banding makesit possible not to release the preloading of the side wall throughpremature unlocking of collars 43.

FIGS. 6 and 7 illustrate the hearth 46 of the embodiment of theinvention. It comprises a main plate 47 provided with a centralconcavity occupied by the cooling boxes 48. Each box 48 comprises awater inlet duct 49 and outlet duct 50.

Similarly to segments 20 of the crucible, it is sought to protect boxes48 against chemical and thermal attack and to provide against opposingelectric arcs occurring between them. They are also coated with ceramic,at least on their upper face (facing the molten bath) 51; the coatingcarries reference 52. And the sharp edges 53 delimiting this upper face51 are rounded, also to a radius of curvature of one to fivemillimetres; the other sharp edges 56 (vertical and delimiting the lowerface 55) may also be rounded or at least chamfered, especially if theside faces 54 and lower faces 55 which they delimit are also coated withceramic.

1. An induction furnace comprising: a side wall comprising verticalside-by-side segments, wherein the segments are coated with ceramic atleast on inner faces and side faces, and at least sharp edges joiningsaid inner faces to said side faces are rounded.
 2. An induction furnaceas in claim 1, further comprising a hearth on which the side wall ispositioned, wherein the hearth is provided with cooling boxes coatedwith ceramic at least on upper faces, and at least sharp edgesdelimiting said upper faces are rounded.
 3. An induction furnace as inclaim 1, wherein the segments are further coated with ceramic on outerfaces.
 4. An induction furnace as in claim 2, wherein the cooling boxesare further coated with ceramic on lower faces and side faces.
 5. Aninduction furnace as in claim 1, wherein the segments comprise lugs forassembly with an upper flange, and the upper flange is coated withceramic at least on one face touching the segments.
 6. An inductionfurnace as in claim 2, wherein the faces of the segments and coolingboxes that are coated with ceramic are joined together by rounded orchamfered edges.
 7. An induction furnace as in claim 1, wherein theceramic is chosen from among mullite, alumina, cordierite, zircon,zirconia, and zirconate.